1. Field of Invention
When lumber from light-colored species such as yellow-poplar, silver maple, or cottonwood is dried by methods known to those skilled in the art at temperatures below 200.degree. F. (e.g. air drying, dehumidification drying, or conventional kiln drying), the color of the lumber is changed very little as a result of the drying. Although kiln drying at atmospheric pressure above 215.degree. F. temperature with air-stream mixtures can produce darkening of the wood surface, in most cases the colored layer is very thin and subsequently planes off in the surfacing of the lumber. The rich brownish tones obtained with wood dried at temperatures above 212.degree. F. enhances the natural figure of the grain and also tends to reduce the conspicuousness of certain blemishes and discolorations.
Two wood species that have particular color problems associated with the dried lumber are black walnut and yellow-poplar. Black walnut lumber, squares, gunstocks, and other products are steamed for several days in below-ground steaming pits or masonry chambers to darken the light-colored sapwood and, thus, tone down the sharp contrast with the dark heartwood. The reduced contrast between heartwood and sapwood in the steamed lumber aids in the uniform finishing of the final product. The steamed lumber is then dried in another piece of equipment. Yellow-poplar contains light-colored sapwood adjacent to a darker greenish cast heartwood. Because of the coloration in the heartwood, the heartwood is rarely used for products with exposed surfaces. The sapwood is generally stained to improve its color quality.
Lumber that is darker and has clearly defined grain has greater market value than the lighter material. Darker material avoids the conspicuous appearances of scratches, dents, and markings especially desired in furniture products. Thus, darker wood such as walnut and cherry have a higher market value than light woods such as yellow-poplar and soft maple.
2. Description of the Prior Art
It is known that various types of wood may be treated to enhance its color characteristics. In the article in the Forest Products Journal, of November, 1964, entitled "Steaming Walnut for Color," it is suggested that the change in color is most probably due to an oxidation of phenolic substances. The article states several conclusions including the fact that the rate of or speed of color change in walnut sapwood increases as the temperature of the stock is increased. The rate increases sharply between 212.degree. F. and 230.degree. F., but above 230.degree. F., no increase was noted.
U.S. Pat. No. 4,127,946 discloses a method and apparatus for drying materials including wood by contacting the materials with a drying gas consisting essentially of internally generated superheated drying steam above 212.degree. F. However, the pressure at which the steam is present in the drying chamber is less than atmospheric.
U.S. Pat. No. 4,233,753 describes a method for preventing the splitting of logs during drying, wherein the logs are placed in a chamber, the chamber sealed and steam is added to about atmospheric pressure.
U.S. Pat. No. 4,246,704 describes a process for drying solid wood in the form of planks by means of super-heated steam. This patent includes the description of the insertion of wood into a dryer followed by the evacuation of the dryer by a vacuum pump, followed by the introduction of steam into the chamber at substantially atmospheric pressure.
U.S. Pat. No. 4,233,752 describes an apparatus and process for treating wood in a hermetically sealed chamber wherein a vacuum is applied, followed by the introduction of a fluid which may be steam, and an increase in pressure.
Reference is also made to U.S. Pat. No. 4,058,906, which describes a process for drying wood, but at sub-atmospheric pressure.
However, none of these processes produce a product as obtained by the process of the present invention.